Compounds having two or more thiol groups in one molecule readily react with epoxy resins, urethane resins or the like by mixing them with such resins and become cured products, and therefore, they have been widely used for sealing materials, coating materials, adhesives, etc. For example, various polythiol-based curing agents are described as low-temperature curing agents on page 204 of “Sosetsu Epokishi Jushi (General Reviews Epoxy Resins)” (Vol. 1, Basic Edition, published on Nov. 19, 2003). Conventional polythiol-based epoxy adhesives with tertiary-amine curing agents, however, have a clear shortcoming that when such a curing agent is mixed with an epoxy compound and a curing assistant at ordinary temperature to form an epoxy resin composition, the gel time of the composition is as short as 3 minutes to 5 minutes and curing is initiated in the course of preparation of the composition, though the curing agent shows excellent curability at low temperatures. Thus, it was rather difficult to control the gel time of adhesives based on epoxy-thiol systems. The gel time is either so long in case of not using curing accelerator, or so short when using curing accelerator. As a result, the working conditions of epoxy-thiol adhesives were restricted.
There are not related references reported about how to control the gel time of the thiol-amine curing system, some patents have mentioned this problem as following:
In US patent No. US 2010/0273940 A1, there is disclosed a curing agent for epoxy resins, which contains a thiol compound having a branch (substituent) on a carbon atom at the α position to a thiol group (—SH), and one compound epoxy resin composition using the curing agent. This invention relates to an epoxy resin curing agent having a pot life from 5 min to 15 min and having storage stability.
In Japanese Patent No. 269203/1996, there is disclosed a thiol group-containing polyether polymer obtained by allowing a halogen-terminated polyether polymer which is obtained by addition of epihalohydrin to a polyol having a polyether moiety in the main chain and having 3 or more hydroxyl groups at the ends to react with an alkali hydrosulfide and/or an alkali polysulfide in amides. It is indicated that an epoxy resin composition containing this polyether polymer has favorable curability, but when this epoxy resin composition is used as a curing agent for epoxy resins, the gel time is so short that curing is initiated while the epoxy resin and a thiol compound that is a curing agent are being mixed with each other, and hence, the working conditions are restricted.
An epoxy resin curing composition using a hetero ring-containing compound, which is described in WO99/54373, has low-temperature curability and rapid curability at ordinary temperature, but the workability is poor because its gel time is short.
In Japanese patent No. 21693/1992, there is disclosed a liquid polysulfide polymer for epoxy resin curing, which is a copolymer obtained by allowing a mixture of a tri- or tetrafunctional alkyl halide and a bifunctional alkyl halide to react with an alkali polysulfide, the amount of said polyfunctional monomers in the polymerization being 20 to 60% by mol based on all the monomers, and which contains 2 to 30% by mass of end thiol groups. It is indicated that by using this liquid sulfide polymer in combination with an amine, the curing rate becomes higher as compared with that of conventional polysulfide polymers. However, the gel time is short, and the resulting cured product has a problem of workability because it develops odor, though it is excellent in impact resistance, chemical resistance, etc.
Thus, despite all these prior arts, there is still a need for a two part epoxy adhesive system, whose gel time is controllable and suitable for satisfying different gel time requirements from customers.
The present invention provides a novel epoxy based adhesive system, which allows a controllable gel time ranging from 5 min to more than 24 hours.